In integrated circuits, there has developed a need for identifying a particular integrated circuit based on some characteristic of the integrated circuit. In the case of memories there is often the capability of repairing a particular memory by using what is commonly known as redundancy. There are included redundant rows and/or columns on the particular integrated circuit memory. If one of the regular rows or columns is defective, a redundant row or column is used to replace the defective one. There has become a desire for knowing if the particular memory is one in which a redundant row or column has replaced a regular row or column. Another identity need is for distinguishing between mask sets. A particular type of memory or other device may be manufacturable from two different mask sets. It is desirable to know from which mask set the device was made. One of the techniques which has been developed for indicating this is having a diode-connected transistor coupled between an input and a power supply terminal. The input can be driven so as to forward bias this diode-connected transistor and current can be detected to identify the integrated circuit as having one characteristic. To identify a second characteristic of the integrated circuit, the diode-connected transistor is decoupled, in the case of redundancy, from the input and/or power supply terminal so that no current flows between the input and the power supply terminal even when the diode-connected transistor is forward biased. For mask set identity, this diode-connected transistor simply is not present so that no current flows. There are any number of different characteristics for which a device may be identified. In the case of redundancy it is desirable to be able to perform the identity function with fuses. In other cases, such as mask identity, the circuit useful for allowing the detection can simply be present or not present. Additionally, more than two characteristics can be distinguished by having more than one input selectively provided with circuitry susceptible to being detected.
This input diode type of identity was fine until ESD protection began including another diode which was connected between VDD and the input terminal. An approach which overcame this new problem is described in U.S. patent application Ser. No. 945,253, now U.S. Pat. No. 4,716,302 entitled "Identity Circuit For An Integrated Circuit Using a Fuse and Transistor Enabled By a Power-On-Reset Signal," Flannagan et al. The approach described in that patent application used the power-on-reset signal for providing signature information. There are potential difficulties in relying on a powr-on-reset signal because the provided power-up of the power supply voltage of a tester of a purchaser of the particular integrated circuit may not result in a signal which is easily detected by the purchaser's tester.